In the OmniVI only TX rf is routed through the LPFs on the LP filter
board; receive does not see the benefit of these LPFs. However BPFs
on the front end board are used for both TX and RX. But in the presence
of a very strong UHF signal, could spurious mixing could occur in the
switching diodes before any filtering? These diodes are used to steer
rf from either the main or AUX antennas to the front end.
Jack K1VT
On Wed, 2007-12-12 at 18:26 -0500, Jack Mandelman wrote:
> While tuning across 20m ssb with my recently acquired Omni VI plus I
> noticed what first appeared to be S5-6 splatter from a nearby signal.
> After closer inspection I realized that it was an FM modulated signal
> centered at about 14.17980 MHz. I then switched the radio to FM and
> was
> able to make out certain softly spoken words, although the signal was
> wideband FM while the Omni VI + is meant to receive narrow band FM.
> The
> FM signal appears to be from a local UHF TV station operating on about
> 635MHz.
>
> To determine if the signal I was hearing was truly on the 20m band I
> listened on both a TS-940S and a TenTec CorsairII. Neither the 940 or
> C2 heard any such signal. Therefore I surmised I was hearing an
> intermod product generated within the Omni VI +. I was able to
> eliminate the intermod by inserting an old Johnson Viking LP filter in
> front of the Omni VI+.
>
> Has anyone encountered or heard of a similar UHF intermod problem with
> the Omni VI?
I can sort of believe it happening. The LO is synthesized in that range
and divided down to 5 MHz.
I'd also check to see if the radio's internal band switched low pass
filters weren't shorted out by faulty relays or diodes. There ought to
be some low pass filter in the front end all the time.
>
> Jack K1VT
> _______________________________________________
73, Jerry, K0CQ
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